In recent years, research and development have been extensively conducted on EL elements. In a basic structure of EL elements, a layer containing a light-emitting substance is provided between a pair of electrodes. By applying voltage to this element, light emission from the light-emitting substance can be obtained.
Since such an EL element is of self-light-emitting type, it is considered that the EL element has advantages over a liquid crystal display in that visibility of pixels is high, backlight is not required, and so on and is therefore suitable as flat panel display elements. In addition, it is also a great advantage that the EL element can be manufactured as a thin and lightweight element. Furthermore, very high speed response is also one of the features of such an element.
Since EL elements can be formed in the form of a film, they make it possible to provide planar light emission. Therefore, large-area elements can be easily formed. This feature is difficult to obtain with point light sources typified by incandescent lamps and LEDs or linear light sources typified by fluorescent lamps. Thus, EL elements also have great potential as planar light sources which can be applied to lighting devices and the like.
EL elements can be broadly classified according to whether the light-emitting substance is an organic compound or an inorganic compound. In the case of an organic EL element in which a layer containing an organic compound as the light-emitting substance is provided between a pair of electrodes, application of a voltage to the light-emitting element causes injection of electrons from the cathode and holes from the anode into the layer containing the organic compound, and thus a current flows. The injected electrons and holes then lead the organic compound to its excited state, whereby light emission is obtained from the excited organic compound.
The excited state of an organic compound can be a singlet excited state and a triplet excited state, and light emission from the singlet excited state (S*) is referred to as fluorescence, and light emission from the triplet excited state (T*) is referred to as phosphorescence.
In improving element characteristics of such a light-emitting element, there are a lot of problems which depend on a substance, and in order to solve the problems, improvement of an element structure, development of a substance, and the like have been carried out. For example, Patent Document 1 discloses an organic light-emitting element including a mixed layer containing an organic low molecular hole-transport substance, an organic low molecular electron-transport substance, and a phosphorescent dopant.